Ultrastable Single-component Material Devices: the Next Frontier for Organic Solar Cells

The long-time operational stability is a central challenge for developing organic solar cells (OSCs) to market readiness. However, many state-of-the-art OSCs based on the bulk-heterojunction concept suffer from stability problems caused by severe morphological changes upon thermal or illumination stress [1]. Single-component materials, enabled by the covalently-bonded structure with donor and acceptor in one molecule, present attractive advantages such as a simplification of device fabrication and stabilization of microstructure [2]. Recently, with the rapid improvement of efficiency from 2-3% to 11.3% for single-component organic solar cells (SCOSCs), this class of materials is getting into the research focus of the OPV community. However, reports on their operational stability are still scanty.In this work, we systematically investigated for the first time the stability under thermal and illumination stress for a series of SCOSCs based on polymeric (SCP3, PBDBPBI-Cl) and molecular (dyad 1, 2, 3, and 4) materials. Under significant thermal stress, double-cable polymer-based SCOSCs exhibited excellent thermal stability with no degradation at 90 oC for 3000 hours. Furthermore, in order to compare polymeric with molecular single component materials, we studied the thermal stability among a series of SCOSCs based on D-A small molecules (dyad 1, 2, and 3) with the same donor and acceptor units but differently long alkyl space linkers. Since macroscopic diffusion of molecules is excluded in these dyads, the length of the spacer can only provide the necessary flexibility for sub-nm rearrangements caused by thermal stress. Interestingly, the single dyads showed a distinctly different behavior: dyad 1 with the shortest linker exhibited the highest thermal stability, while dyad 3 with the longest linker showed the relatively lowest thermal stability [3]. This highlights the need for further in-depth studies on optimizing the spacer length in parallel for performance and stability. Moreover, dyad 1-based SCOSCs exhibited exceptional illumination stability, retaining 98% of the initial PCE under concentrated light equal to 7.5-suns for over 1000 hours, which is among the best values for solution-processed OSCs. Based on the outstanding stability, SCOSCs could be an ideal candidate to study the ultimate stability under extremely rugged conditions such as high temperature and concentrated light. Since the morphological evolution is excluded, SCOSCs could serve as a model system to selectively study interface degradation. Shortly, SCOSCs are predicted to see a prospective renaissance with efficiencies over 10% and a lifetime of over 20 years, thus closing the gap towards industrial applications